Add Postgres repo type

[PossibleLlama]

Is your feature request related to a problem? Please describe.
To enable users to store the backend wherever they want (including the cloud), we could use a common database from which users can spin up an instance as they want, wherever they want.

Describe the solution you'd like
A PostgresQL repository type that can sit next to the bolt type, including all of the expected functionality that the bolt repo currently supports.

Additional context